Drilling wells for oil and gas production conventionally employs longitudinally extending sections, or so-called “strings,” of drill pipe to which, at one end, is secured a drill bit of a larger diameter. After a selected portion of the borehole has been drilled, a string of tubular members of lesser diameter than the borehole, known as casing, is placed in the borehole. Subsequently, the annulus between the wall of the borehole and the outside of the casing is filled with cement. Therefore, drilling and casing according to the conventional process typically requires sequentially drilling the borehole using drill string with a drill bit attached thereto, removing the drill string and drill bit from the borehole, and disposing and cementing a casing into the borehole. Further, often after a section of the borehole is lined with casing and cemented, additional drilling beyond the end of the casing or through a sidewall of the casing may be desired. In some instances, a string of smaller tubular members, known as a liner string, is run and cemented within previously run casing. As used herein, the term “casing” includes tubular members in the form of liners.
Because sequential drilling and running a casing or liner string may be time consuming and costly, some approaches have been developed to increase efficiency, including the use of reamer shoes disposed on the end of a casing string and drilling with the casing itself. Reamer shoes employ cutting elements on the leading end that can drill through modest obstructions and irregularities within a borehole that has been previously drilled, facilitating running of a casing string and ensuring adequate well bore diameter for subsequent cementing. Reamer shoes also include an end section manufactured from a material that is readily drillable by drill bits. Accordingly, when cemented into place, reamer shoes usually pose no difficulty to a subsequent drill bit to drill through. For instance, U.S. Pat. No. 6,062,326 to Strong et al. discloses a casing shoe or reamer shoe in which the central portion thereof may be configured to be drilled through. However, the use of reamer shoes requires the retrieval of the drill bit and drill string used to drill the borehole before the casing string with the reamer shoe is run into the borehole.
Drilling with casing is effected using a specially designed drill bit, termed a “casing bit,” attached to the end of the casing string. The casing bit functions not only to drill the earth formation, but also to guide the casing into the borehole. The casing string is, thus, run into the borehole as it is drilled by the casing bit, eliminating the necessity of retrieving a drill string and drill bit after reaching a target depth where cementing is desired. While this approach greatly increases the efficiency of the drilling procedure, further drilling to a greater depth must pass through or around the casing bit attached to the end of the casing string.
In the case of a casing shoe, reamer shoe or casing bit that is drillable, further drilling may be accomplished with a smaller diameter drill bit and casing string attached thereto that passes through the interior of the first casing string to drill the further section of the borehole beyond the previously attained depth. Of course, cementing and further drilling may be repeated as necessary, with correspondingly smaller and smaller tubular components, until the desired depth of the wellbore is achieved.
However, where a conventional drill bit is employed and it is desired to leave the bit in the well bore, further drilling may be difficult, as conventional drill bits are required to remove rock from formations and, accordingly, often include very drilling-resistant, robust structures typically manufactured from materials such as tungsten carbide, polycrystalline diamond, or steel. Attempting to drill through a conventional drill bit affixed to the end of a casing may result in damage to the subsequent drill bit and bottom-hole assembly deployed. It may be possible to drill through casing above a conventional drill bit with special tools known as mills, but these tools are generally unable to penetrate rock formations effectively to any great distance and, so, would have to be retrieved or “tripped” from the borehole and replaced with a drill bit. In this case, the time and expense saved by drilling with casing would have been lost.
To enable effective drilling of casing and casing-associated components manufactured from robust, relatively inexpensive and drillable iron-based materials such as, for example, high-strength alloy steels, which are generally non-drillable by diamond cutting elements as well as subsequent drilling through the adjacent formation, it would be desirable to have a drill bit or tool offering the capability of drilling through such casing or casing-associated components, while at the same time offering the subterranean drilling capabilities of a conventional drill bit or tool employing superabrasive cutting elements.